By Kim Bo-eunTunnel excavation has been detected at North Korea's Punggye-ri nuclear test site, a website analyzing North Korea said Friday."Recent commercial satellite imagery of North Korea's Punggye-ri nuclear test site indicates that the North Portal, where the last five nuclear tests were conducted, remains dormant and that tunnel excavation has been stepped up at the West Portal," 38 North posted.It said mining carts and personnel were seen around the West Portal throughout December and the pile of material excavated from the mines significantly increased.The post also said there were between 100 and 120 personnel observed in seven different formations on Dec. 28. It said it is rare to see people in this area and the purpose of their activities is unknown.In addition, 38 North-identified nine mining carts at the West Portal, as well as new rails on top of excavated material."These activities underscore North Korea's continued efforts to maintain the Punggye-ri site's potential for future nuclear testing," it said.Pyongyang's last nuclear test was in September last year.In his New Year address, North Korean leader Kim Jong-un warned that the nuclear button is always on his desk.However, Kim also expressed willingness to send a delegation to the PyeongChang Winter Olympics.South Korea welcomed the olive branch and the two held high-level talks on Tuesday, at which an agreement was reached to send a large North Korean team to the Games and for inter-Korean military talks to resume.But the North's chief delegate, Committee for the Peaceful Reunification of the Country Chairman Ri Son-gwon, expressed discomfort over the South's calls for denuclearization.The Koreas are yet to arrange follow-up talks.The latest developments have eased military tension on the Korean Peninsula. International organizations, including the United Nations Security Council, and the U.S., China and Russia have welcomed the progress in inter-Korean dialogue.South Korea will maintain close communication with the U.S. to discuss follow-up measures. Following the inter-Korean talks, South Korean President Moon Jae-in held a telephone conversation with U.S. President Donald Trump, Wednesday, in which Trump confirmed that Washington will not take military action against Pyongyang as long as the Koreas are talking.